It is well known that two cycle gasoline engines have not been widely used in automobiles and trucks due, in part at least, to their poor fuel economy and emissions caused by the carryover of fuel to the exhaust gases during the scavenging process. These problems, which are compounded by excessive exhaust smoke caused by the addition of lubricant to the fuel, are among the known shortcomings of the carbureted and crankcase scavenged two cycle engines that are commonly used in lawn mowers, snowmobiles, chain saws, motorcycles and outboard boats, among other uses. On the other hand, two cycle diesel engines in which the fuel is injected into the cylinders near the end of the compression stroke and which generally use blower scavenging with air unmixed with oil have avoided these problems and have been widely used in trucks, locomotives, ships, power plants, oil rigs, inboard boats and various industrial applications.
Recently, interest has developed in two cycle engines using low pressure cylinder fuel injection of gasoline to avoid the problem of fuel carryover into the exhaust gases. Pressure lubrication has also been used to reduce oil carryover. There remains, however, an additional problem of exhaust emission control which bears upon use in automotive vehicles with catalytic converters. The excess air utilized for scavenging of two cycle engine cylinders mixes with the burned gas from combustion in the cylinders and cools the exhaust gases so that the warm-up time for an exhaust catalytic converter becomes excessive, delaying the time to establish adequate emission control after startup. In addition, extended idle or low power operation may cool the converter below its effective operating temperature.
In connection with conventional two cycle engines, of the carbureted type for example, Sato and Nakano in SAE paper 871653 published Sept. 14, 1987 disclosed means for discharging exhaust gas flowing from the cylinder into three sequentially opening exhaust ports for test purposes. With this separation of the exhaust gases, the scavenging mixture discharged through the last to open of the three ports was largely air/fuel mixture, as was shown by running a four cycle engine solely with this segregated exhaust gas as the inlet charge. However, this SAE paper does not suggest any use for selectively separating the exhaust gases in an air scavenged two cycle engine where relatively little unburned fuel remains in the exhaust gases.